Exterior Gateway Protocols: EGP, BGP-4, CIDR: Brief …€¦ · · 2004-02-05Exterior Gateway Protocols: EGP, BGP-4, CIDR: Brief Version ... Overview. Rensselaer ... Border Gateway

Shivkumar KalyanaramanRensselaer Polytechnic Institute

1

Exterior Gateway Protocols: EGP, BGP-4, CIDR:

Brief Version


[email protected] http://www.ecse.rpi.edu/Homepages/shivkuma

Based in part upon slides of Tim Griffin (AT&T), Ion Stoica (UCB), J. Kurose (U Mass), Noel Chiappa(MIT)

http://www.ecse.rpi.edu/Homepages/shivkuma


2

Cores, Peers, and the limit of default routesAutonomous systems & EGPBGP4CIDR: reducing router table sizesRefs: Chap 10,14,15. Books: “Routing in Internet” by Huitema, “Interconnections” by Perlman, “BGP4” by Stewart, Sam Halabi, Danny McPherson, Internet Routing Architectures Reading: Geoff Huston, Commentary on Inter-domain Routing in the Internet Reference: BGP-4 Standards Document: In TXT Reading: Norton, Internet Service Providers and PeeringReading: Labovitz et al, Delayed Internet Routing ConvergenceReference: Paxson, End-to-End Routing Behavior in the Internet, Reading: Interdomain Routing: Additional Notes: In PDF | In MS Word Reference Site: Griffin, Interdomain Routing Links

Overview


3

Intra-AS and Inter-AS routing

inter-AS, intra-ASrouting in

gateway A.c

network layerlink layer

physical layer

a

b

b

aaC

A

Bd

Gateways:•perform inter-ASrouting amongst themselves•perform intra-ASrouters with other routers in their AS

A.cA.a

C.bB.a

cb

c


4

History of Inter-Domain Routing & EGP


5

History: Default Routes: limitsDefault routes => partial informationRouters/hosts w/ default routes rely on other routers to complete the picture.In general routing “signposts” should be:

Consistent, I.e., if packet is sent off in one direction then another direction should not be more optimal.Complete, I.e., should be able to reach all destinations


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CoreA small set of routers that have consistent & complete information about all destinations.Non-core routers can have partial information provided they point default routes to the core

Partial info allows site administrators to make local routing changes independently.

CORE

. . .S1 S2 Sm


7

Peer BackbonesInitially NSFNET: only 1 link to ARPANETAddition of multiple links => multiple possible routes => need for dynamic routing

Today there are over 30 backbones!Routing protocol at cores/peers: GGP -> EGP-> BGP-4

NSFNET

S1 S2 Sm. . . s1

Peering LinkARPANet

s2 sn. . .


8

Exterior Gateway Protocol (EGP)A mechanism that allows non-core routers to learn routes from core (external routes) routers so that they can choose optimal backbone routes

A mechanism for non-core routers to inform core routers about hidden networks (internal routes)

Autonomous System (AS) has the responsibility of advertising reachability info to other ASs.


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Purpose of EGP

R border router

internal router

EGPR2

R1

R3

A

AS1

AS2

you can reachnet A via me

traffic to A

table at R1:dest next hopA R2

Share connectivity information across ASes


10

EGP OperationNeighbor Acquisition: Reliable 2-way handshakeNeighbor Reachability:

Hellos: j out of m hellos OK => Neighbor UPk out of n hellos NOT OK => Neighbor DOWN

Updates/Queries:EGP is an incremental protocol. New info => send updatesEach router can query neighbors as wellReachability advertized; metrics ignoredRequires a tree topology of ASes to avoid loops (see next slide)


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Why EGP Requires a Tree Structure..


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EGP weaknessesEGP does not interpret the distance metrics in routing update messages => cannot be compute shorter of two routes

As a result it restricts the topology to a tree structure, with the core as the root

Rapid growth => many networks may be temporarily unreachableOnly one path to destination => no load sharing

Need new protocol => BGP-4


13

The Current Stage for Inter-Domain Routing: ASes & Policy Routing

Scenarios


14

Today’s Big Picture

Large ISPLarge ISP

Dial-UpISP

AccessNetwork

Small ISP

Stub

Stub Stub

Large number of diverse networks


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Autonomous Systems (ASes)An autonomous system is an autonomous routing

domain that has been assigned an Autonomous System Number (ASN).

All parts within an AS remain connected.

RFC 1930: Guidelines for creation, selection, and registration of an Autonomous System

… the administration of an AS appears to other ASes to have a single coherent interior routing plan and presents a consistent picture of what networks are reachable through it.


16

Autonomous System(AS)

An autonomous system (AS) is a network under a single administrative control

An AS owns an IP prefixEvery AS has a unique AS number

ASes need to inter-network themselves to form a single virtual global network

Need a common protocol for communicationI.e. BGP-4


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IP Address Allocation and Assignment: Internet Registries

IANAwww.iana.org

RFC 2050 - Internet Registry IP Allocation Guidelines RFC 1918 - Address Allocation for Private Internets RFC 1518 - An Architecture for IP Address Allocation with CIDR

ARINwww.arin.org

APNICwww.apnic.org

RIPEwww.ripe.org

Allocate to National and local registries and ISPsAddresses assigned to customers by ISPs


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AS Numbers (ASNs)

ASNs are 16 bit values.64512 through 65535 are “private”

Currently over 11,000 in use.• Genuity: 1 • MIT: 3• Harvard: 11• UC San Diego: 7377• AT&T: 7018, 6341, 5074, … • UUNET: 701, 702, 284, 12199, …• Sprint: 1239, 1240, 6211, 6242, …• …

ASNs represent units of routing policy


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Internet AS Map: caida.org


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Which Routers do Inter-AS routing?

R border router internal router

BGPR2

R1

R3AS1

AS2

Two types of routersBorder router(Edge), Internal router(Core)

Two border routers of different ASes will have a BGP session


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Requirements for Inter-AS RoutingShould scale for the size of the global Internet.

Focus on reachability, not optimalityUse address aggregation techniques to minimize core routing table sizes and associated control trafficAt the same time, it should allow flexibility in topological structure (eg: don’t restrict to trees etc)

Allow policy-based routing between autonomous systemsPolicy refers to arbitrary preference among a menu of available routes (based upon routes’ attributes)Fully distributed routing (as opposed to a signaled approach) is the only possibility.Extensible to meet the demands for newer policies.


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Policy Routing: Nontransit vs Transit ASes

ISP 1ISP 2

NET ATraffic NEVER flows from ISP 1through NET A to ISP 2

Internet Serviceproviders (ISPs)have transitnetworks

Nontransit ASmight be a corporateor campus network.Could be a “content provider”


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Policy Routing: Selective Transit

NET BNET C

NET A provides transitbetween NET B and NET Cand between NET D and NET C

NET A

NET D

NET A DOES NOTprovide transitBetween NET D and NET B

Most transit ASes allow only selective transitkey impact of commercialization


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Policy Routing: Customers & Providers

provider

customer

IP trafficprovider customer

Customer pays provider for access to the Internet


25

Policy Routing: Customer-Provider Hierarchy

IP trafficprovider customer


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Policy Routing: The Peering Relationship

Peers provide transit between their respective customers

Peers do not provide transit between peers

Peers (often) do not exchange $$$

peer peer

customerprovider

trafficallowed

traffic NOTallowed


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Peering WarsPeer Don’t Peer

Reduces upstream transit costsCan increase end-to-end performanceMay be the only way to connect your customers to some part of the Internet (“Tier 1”)

You would rather have customersPeers are usually your competitionPeering relationships may require periodic renegotiation

Peering struggles are by far the most contentious issues in the ISP world!Peering agreements are often confidential.


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BGP-4 Design


29

Recall: Distributed Routing Techniques

Link State Vectoring

Topology information is flooded within the routing domainBest end-to-end paths are computed locally at each router. Best end-to-end paths determine next-hops.Based on minimizing some notion of distanceWorks only if policy is sharedand uniformExamples: OSPF, IS-IS

Each router knows little about network topologyOnly best next-hops are chosen by each router for each destination network. Best end-to-end paths result from composition of all next-hop choicesDoes not require any notion of distanceDoes not require uniform policies at all routersExamples: RIP, BGP


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BGP-4BGP = Border Gateway Protocol Is a Policy-Based routing protocol Is the de facto EGP of today’s global InternetRelatively simple protocol, but configuration is complex and the entire world can see, and be impacted by, your mistakes.

• 1989 : BGP-1 [RFC 1105]– Replacement for EGP (1984, RFC 904)

• 1990 : BGP-2 [RFC 1163]• 1991 : BGP-3 [RFC 1267]• 1995 : BGP-4 [RFC 1771]

– Support for Classless Interdomain Routing (CIDR)


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BGP Operations (Simplified)

Establish session onTCP port 179

Exchange allactive routes

Exchange incrementalupdates

AS1

AS2

BGP session

While connection is ALIVE exchangeroute UPDATE messages


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Four Types of BGP Messages

Open : Establish a peering session. Keep Alive : Handshake at regular intervals. Notification : Shuts down a peering session. Update : Announcing new routes or withdrawingpreviously announced routes.

announcement =

prefix + attributes values


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Border Gateway Protocol (BGP)Allows arbitrary AS topologiesUses a path-vector concept to help prevent routing loops in complex topologiesFor inter-domain routing: shortest path may not be preferred for policy, security, cost reasons.

Different routers have different preferences (policy) => as packet goes thru network it will encounter different policies=> Bellman-Ford or Dijkstra don’t work!

Soln: BGP allows attributes for AS and paths which could include policies (policy-basedrouting).


34

BGP (Cont’d)Consistency criterion: When a BGP Speaker A advertises a prefix to its B that it has a path to IP prefix C …

… B can be certain that A is actively using that AS-path to reach that destination

BGP uses TCP between 2 peers (reliability)Exchange entire BGP table first (50K+ routes!)Later exchanges only incremental updatesApplication (BGP)-level keepalive messages

Interior and exterior peers: need to exchangereachability information among interior peers before updating intra-AS forwarding table.


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Two Types of BGP Neighbor Relationships

• External Neighbor (eBGP) in a different Autonomous Systems

• Internal Neighbor (iBGP) in the same Autonomous System AS1

AS2

eBGP

iBGP

iBGP is routed (using IGP!)


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I-BGP and E-BGP

R border router

internal router

R1

AS1

R4R5

B

AS3

E-BGP

R2R3

AAS2 announce B

IGP: Interior Gateway Protocol.Examples: IS-IS, OSPF

I-BGP

IGP


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I-BGP vs IGP Why is IGP (OSPF, ISIS) not used ?

In large ASs full route table is very large (100K routes!)Rate of change of routes is frequentTremendous amount of control trafficNot to mention Dijkstra computation being evoked for any change…BGP policy information may be lost

I-BGP :Within an ASSame protocol/state machines as EBGP But different rules about advertising prefixes


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IBGP vs EBGPI-BGP nodes: typically ABRs, or other nodes where default routes terminateI-BGP peering sessions between every pair of routers within an AS: full mesh.

A

B

D

C

Physical link

IBGP session

AS1


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IBGP Peers: Fully MeshedeBGP update

iBGP updates

IBGP is needed to avoid routing loops within an ASFull Mesh =>

Independent of physical connectivity.Single link may see same update multiple times!

IBGP neighbors do not announce routes received via iBGP to other iBGPneighbors.


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IBGP Scaling: Route ReflectionAdd hierarchy to I-BGP Route reflector: A router whose BGP implementation supports the re-advertisement of routes between I-BGP neighborsRoute reflector client: A router which depends on route reflector to re-advertise its routes to entire AS and learn routes from the route reflector


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Route Reflection

RR-C1

RR-C2

RR1

RR2

RR3

RR-C3

RR-C4

AS1

AS2

ER10.0.0.0/24

128.23.0.0/16

EBGP

IBGP


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AS ConfederationsDivide and conquer: Divides a large AS into sub-ASs

AS-112

10

14

11

13R1

R2

Sub-AS


43

BGP-4: Support for Scaling and Address Management


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CIDRShortage of class Bs => give out a set of class Cs instead of one class B address

Problem: every class C n/w needs a routing entry !

Solution: Classless Inter-domain Routing (CIDR). Also called “supernetting”Key: allocate addresses such that they can be summarized, I.e., contiguously.

Share same higher order bits (I.e. prefix)Routing tables and protocols must be capable of carrying a subnet mask. Notation: 128.13.0/23

When an IP address matches multiple entries (eg194.0.22.1), choose the one which had the longest mask (“longest-prefix match”)


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Inter-domain Routing Without CIDR

ServiceProvider

Global InternetRouting

Mesh

204.71.0.0204.71.1.0204.71.2.0…...…….

204.71.0.0204.71.1.0204.71.2.0

204.71.255.0

…...…….204.71.255.0

Inter-domain Routing With CIDR

ServiceProvider

204.71.0.0204.71.1.0204.71.2.0…...…….

Global InternetRouting

Mesh

204.71.0.0/16

204.71.255.0


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RFC 1519: Classless Inter-Domain Routing (CIDR)

Pre-CIDR: Network ID ended on 8-, 16, 24- bit boundaryCIDR: Network ID can end at any bit boundaryIP Address : 12.4.0.0 IP Mask: 255.254.0.0

00001100 00000100 00000000 00000000

11111111 11111110 00000000 00000000

Network Prefix

Address

Mask

for hosts

Usually written as 12.4.0.0/15, a.k.a “supernetting”


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Longest Prefix Match (Classless) Forwarding

Destination =12.5.9.16-------------------------------

payload

Prefix Interface Next Hop

12.0.0.0/8 10.14.22.19 ATM 5/0/8

12.4.0.0/15

12.5.8.0/23 attached

Ethernet 0/1/3

Serial 1/0/7

10.1.3.77

IP Forwarding Table

0.0.0.0/0 10.14.11.33 ATM 5/0/9 OK

better

even better

best!


48

ISP3

AS1128.40/16140.127/16

Link A

Link B

ISP1128.32/11

ISP2140.64/10

128.40/16

140.127/16

ISP2ISP1

ORIGIN AS

Next Hop

Prefix

ISP2140.64/10ISP1128.32/11

Table at ISP3CIDR at Work, No load balancing


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CIDR Subverted for Load Balancing

ISP3

AS1128.40/16140.127/16

Link A

Link B

ISP1128.32/11

ISP2140.64/10

140.255.20/24, 128.40/16

128.42.10/24, 140.127/16

AS1AS1ISP2ISP1

ORIGIN AS

Next Hop

Prefix

ISP2128.42.10/24ISP1140.255.20/24ISP2140.64/10ISP1128.32/11

Table at ISP3


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Deaggregation + Multihoming

AS 1

customerAS 2

provider

AS 3provider

12.2.0.0/16

If AS 1 doesnot announce themore specific prefix,then most traffic to AS 2 will go through AS 3 because it is a longer match

12.2.0.0/1612.2.0.0/1612.0.0.0/8

AS 2 is “punching a hole” in the CIDR block of AS 1=> subverts CIDR


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Policy Routing in BGP-4


52

What is Routing PolicyPolicy refers to arbitrary preference among a menu of available routes

Public description of the relationship between external BGP peersCan also describe internal BGP peer relationship

BGP Hook: policy routing choice based upon routes’ attributes


53

How to do policy routing?192.0.2.0/24pick me!

192.0.2.0/24pick me!

192.0.2.0/24pick me!

192.0.2.0/24pick me!

Given multipleroutes to the sameprefix, a BGP speakermust pick at mostone best route based upon routes’ attributes


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BGP Policy Knob: Attributes

Value Code Reference----- --------------------------------- ---------

1 ORIGIN [RFC1771]2 AS_PATH [RFC1771]3 NEXT_HOP [RFC1771]4 MULTI_EXIT_DISC [RFC1771]5 LOCAL_PREF [RFC1771]6 ATOMIC_AGGREGATE [RFC1771]7 AGGREGATOR [RFC1771]8 COMMUNITY [RFC1997]9 ORIGINATOR_ID [RFC2796]

10 CLUSTER_LIST [RFC2796]11 DPA [Chen]12 ADVERTISER [RFC1863]13 RCID_PATH / CLUSTER_ID [RFC1863]14 MP_REACH_NLRI [RFC2283] 15 MP_UNREACH_NLRI [RFC2283] 16 EXTENDED COMMUNITIES [Rosen]

...255 reserved for development

We will cover a subset of these attributes

Not all attributesneed to be present inevery announcement

From IANA: http://www.iana.org/assignments/bgp-parameters


55

Import and Export PoliciesFor inbound traffic

Filter outbound routesTweak attributes on outbound routes in the hope of influencing your neighbor’s best route selection

For outbound trafficFilter inbound routesTweak attributes on inbound routes to influence best route selection

outboundroutes

inboundroutes

inboundtraffic

outboundtraffic

In general, an AS has morecontrol over outbound traffic


56

BGP Route Processing

Best RouteSelection

Apply ImportPolicies

Best Route Table

Apply ExportPolicies

Install forwardingEntries for bestRoutes.

Apply Policy =filter routes & tweak

Based onAttributeValues

IP Forwarding Table

Apply Policy =filter routes & tweak attributes

BestRoutes

TransmitBGP Updates

ReceiveBGPUpdates attributes


57

Policy Implementation Flow

MainBGPRIB

AdjRIBOut

Outgo-ing

AdjRIBIn

Incom-ing

MainRIB/FIB

IGPs

Static&

HWInfo


58

Conceptual Model of BGP OperationRIB : Routing Information BaseAdj-RIB-In: Prefixes learned from neighbors. As many Adj-RIB-In as there are peersLoc-RIB: Prefixes selected for local use after analyzing Adj-RIB-Ins. This RIB is advertised internally.Adj-RIB-Out : Stores prefixes advertised to a particular neighbor. As many Adj-RIB-Out as there are neighbors


59

BGP-4 Messages and Route Attributes


60

UPDATE message in BGPPrimary message between two BGP speakers.Used to advertise/withdraw IP prefixes (NLRI)Path attributes field : unique to BGP

Apply to all prefixes specified in NLRI fieldOptional vs Well-known; Transitive vs Non-transitive

2 octets

Withdrawn Routes Length

Withdrawn Routes (variable length)

Total Path Attributes Length

Path Attributes (variable length)

Network Layer Reachability Info. (NLRI: variable length)


61

Path Attributes: ORIGINORIGIN:

Describes how a prefix came to BGP at the origin ASPrefixes are learned from a source and “injected” into BGP:Directly connected interfaces, manually configured static routes, dynamic IGP or EGP

Values: IGP (EGP): Prefix learnt from IGP (EGP)INCOMPLETE: Static routes


62

Path Attributes: AS-PATHList of ASs thru which the prefix announcement has passed. AS on path adds ASN to AS-PATHEg: 138.39.0.0/16 originates at AS1 and is advertised to AS3 via AS2.Eg: AS-SEQUENCE: “100 200”Used for loop detection and path selection

AS1(100)

AS2(200)

AS3(15)

138.39.0.0/16


63

Traffic Often Follows ASPATH

AS 4

135.207.0.0/16ASPATH = 3 2 1

AS 3AS 1135.207.0.0/16

AS 2

IP Packet Dest =135.207.44.66


64

… But It Might Not

AS 4AS 3AS 2AS 1135.207.0.0/16

135.207.0.0/16ASPATH = 3 2 1

IP Packet Dest =135.207.44.66

AS 5

135.207.44.0/25ASPATH = 5

135.207.44.0/25

AS 2 filters allsubnets with maskslonger than /24

From AS 4, it may look like this packet will take path 3 2 1, but it actually takes path 3 2 5

135.207.0.0/16ASPATH = 1


65

Shorter AS-PATH Doesn’t Mean Shorter # Hops

AS 4

AS 3

AS 2

AS 1

BGP says that path 4 1 is betterthan path 3 2 1

Duh!


66

ASPATH Padding: Shed inbound traffic

Padding will (usually) force inbound traffic from AS 1to take primary link

AS 1

192.0.2.0/24ASPATH = 2 2 2

customerAS 2

provider

192.0.2.0/24

backupprimary

192.0.2.0/24ASPATH = 2


67

Load-Balancing Knobs in BGP

Local Preference

AS1 AS2

MED

LOCAL-PREF: outbound traffic, local preference (box-level knob)MED: Inbound-traffic, typically from the same ISP (link-level knob)


68

Path Attribute: LOCAL-PREFLocally configured indication about which path is preferred to exit the AS in order to reach a certain network. Default value = 100. Higher is better.


69

Hot Potato Routing: Closest Egress Point

192.44.78.0/24

15 56 IGP distances

egress 1 egress 2

This Router has two BGP routes to 192.44.78.0/24.

Hot potato: get traffic off of your network as Soon as possible. Go for egress 1!


70

Getting Burned by the Hot Potato

15 56

172865High bandwidth

Provider backbone

Low b/wcustomer backbone

HeavyContent

Web Farm

SFF NYC

San Diego

Many customers want their provider to carry the bits!

tiny http requesthuge http reply


71

Attributes: MULTI-EXIT Discriminator

Also called METRIC or MED Attribute. Lower is betterAS1:multihomed customer. AS2 (provider) includes MED to AS1AS1 chooses which link (NEXTHOP) to useEg: traffic to AS3 can go thru Link1, and AS2 thru Link2

AS1 AS2

AS3

AS4

Link A

Link B


72

MEDs Can Export Internal Instability

15

172865 Heavy

Content Web Farm

192.44.78.0/24

192.44.78.0/24MED = 15

192.44.78.0/24MED = 56 OR 10

5610

FLAP

FLAP

FLAP

FLAP

FLAPFLAP


73

How Can Routes be Colored?BGP Communities

A community value is 32 bits

By convention, first 16 bits is ASN indicating who is giving itan interpretation

communitynumber

• Used within and betweenASes

• The set of ASes must agree on how to interpret the community value• Very powerful BECAUSE it has no (predefined) meaning

Community Attribute = a list of community values.(So one route can belong to multiple communities)

Two reserved communities

no_advertise 0xFFFFFF02: don’t pass to BGP neighborsno_export = 0xFFFFFF01: don’t export out of AS

RFC 1997 (August 1996)


74

Communities Example1:100

Customer routes1:200

Peer routes1:300

Provider Routes

To Customers1:100, 1:200, 1:300

To Peers1:100

To Providers1:100

Import Export

AS 1


75

BGP Route Selection ProcessSeries of tie-breaker decisions...If NEXTHOP is inaccessible do not consider the route. Prefer largest LOCAL-PREFIf same LOCAL-PREF prefer the shortest AS-PATH. If all paths are external prefer the lowest ORIGIN code(IGP<EGP<INCOMPLETE). If ORIGIN codes are the same prefer the lowest MED.If MED is same, prefer min-cost NEXT-HOPIf routes learned from EBGP or IBGP, prefer paths learnt from EBGPFinal tie-break: Prefer the route with I-BGP ID (IP address)


76

Route Selection Summary

Highest Local Preference Enforce relationships

Shortest ASPATH

Lowest MED

i-BGP < e-BGP

Lowest IGP cost to BGP egress

traffic engineering

Lowest router ID Throw up hands andbreak ties


77

Caveat

• BGP is not guaranteed to converge on a stable routing. Policy interactions could lead to “livelock” protocol oscillations.

See “Persistent Route Oscillations in Inter-domain Routing” by K. Varadhan, R.

Govindan, and D. Estrin. ISI report, 1996

• Corollary: BGP is not guaranteed to recover from network failures.


78

BGP Table Growth

Thanks: Geoff Huston. http://www.telstra.net/ops/bgptable.html


79

ASNs Growth

From: Geoff Huston. http://www.telstra.net/ops


80

BGP Updates: Mostly Stable…

Typically, 80% ofthe updates are for less than 5% Of the prefixes.

Most prefixes are stable most of the time. On this day, about 83% of the prefixes were not updated.

Percent of BGP table prefixes

Thanks to Madanlal Musuvathi for this plot. Data source: RIPE NCC


81

Route Flap Dampening

route dampenedfor nearly 1 hour

penalty for each flap = 1000


82

BGP Convergence: How Long Does BGP Take to Adapt to Changes?

0

10

20

30

40

50

60

70

80

90

100

0 20 40 60 80 100 120 140 160

Seconds Until Convergence

Cum

ulat

ive

Perc

enta

ge o

f Eve

nts

Tup

Tshort

Tlong

Tdow n

From: Abha Ahuja and Craig Labovitz


83

Summary

BGP is a fairly simple protocol …… but it is not easy to configureBGP is running on more than 100K routers making it one of world’s largest and most visible distributed systemsGlobal dynamics and scaling principles are still not well understoodTraffic Engineering hacked in as an afterthought…

Documents

Exterior Gateway Protocols: EGP, BGP-4, CIDR: Brief …€¦ · · 2004-02-05Exterior Gateway Protocols: EGP, BGP-4, CIDR: Brief Version ... Overview. Rensselaer ... Border Gateway